Are you sure this is a memory leakage? 4 to 8% memory usage is low.
Linux systems tent to build a cache in memory, hence the increase after a reboot.
Try to wait a little longer and see if it reaches 100%, then you might have a memory leak.

If you want to investigate, put an entry in crontab to list meminfo every hour or so and dump that in a file (or better, create a domoticz device). simplest to issue:
egrep 'MemAvailable' /proc/meminfo | awk '{ print (1-($2 / 947732)) * 100}'
Do the same for the domoticz process:
ps aux|grep "domoticz/domoticz"|grep -v grep| awk '{print $4}'
And see what happens.

and that is close to the output of the suggested ps command.
My normal memory usage is stable, the only one growing is the memory use of Domoticz.
Domoticz is restarted every morning by logrotate.
The Domoticz log is going to a tmpfs and is growing fast, so I have to do that every day.
Is it possible to logrotate without a restart of Domoticz?
Below the picture from the last year.
As you can see I updated Domoticz to the latest stable in the beginning of August.

and that is close to the output of the suggested ps command.
My normal memory usage is stable, the only one growing is the memory use of Domoticz.
Domoticz is restarted every morning by logrotate.
The Domoticz log is going to a tmpfs and is growing fast, so I have to do that every day.
Is it possible to logrotate without a restart of Domoticz?
Below the picture from the last year.
As you can see I updated Domoticz to the latest stable in the beginning of August.

chart (1).png

You can send kill -HUP signal to only reload the logging system, with the supplied init file you can simply call domoticz.sh reload.
What / how many hardware are you using? It could help to disable them one at a time to see if the usage will no longer increase and report back here, so we can take a look at the specific hardware's code. RFXCom and Razberry are unlikely to be the cause, I also use those, so these can be skipped (or you should try disabling them at last).

The system is a production system, so I cannot, for longer time, disable hardware.
The hardware I am using is RFXCOM, RaZberry, Weather, Motherboard, SolarEdge, Milight LAN, System Alive Checker, WOL, and a lot of dummies for all kind of sensors, like this one.
I will try the -HUP signal

I run a almost empty TEST system with only Motherboard and some dummy sensors.
That system does not consume extra memory.
I investigate further to find the "bad apple"

I have a slave system with other hardware. I will implement the same Domoticz Memory usage dummy sensor and see if I can narrow it down.

Currently Domoticz memory usage has grown to more then 11 %.
I also have a production slave system with less hardware.
The slave is also growing in Domoticz memory usage but at regular intervals like 4-5 hours with a step.
Unfortunately it is not a fixed interval.
I also have a TEST system with only the motherboard and 2 dummy sensors.
The TEST system is NOT growing in memory consumption.

So to me it looks like a real problem. The only challenge is how to discover what is causing this behaviour.

I keep it running for a few more days. After that I revert back to stopping and starting Domoticz every day.

I agree, there is a slight memory leak somewhere. I run Domoticz on a Pi-3, and the RAM usage graph shows a gradual but consistent increase.
The graph shows a sharp dip, this is where I rebooted the Pi (for other reasons).
As you can see the memory usage is gradually increasing.
I will keep an eye on it.

Unfortunately for you (fortunately for me) I cannot confirm. With the previous beta, I did see a gradual increase during the day for the entire but the usage would drop after midnight to the base (not related to logrotate). The domoticz process usage has been very stable for me around 4.5%.

@manjh: What HW you have running? Scripts, blocky etc? This info could be helpful for developers.

Memory usage increases from 8% to 10% over the past four days. I understand that 10% is nothing to worry about, but the slope of the graph is: is goes up consistently.
I use the Pi for Domoticz only, run lua scripts (not Blockly).

Memory usage increases from 8% to 10% over the past four days. I understand that 10% is nothing to worry about, but the slope of the graph is: is goes up consistently.
I use the Pi for Domoticz only, run lua scripts (not Blockly).

Are you sure there are no errors in de log which might indicate that Lua scripts never finishes? Something like "warning script took more than 10 seconds to execute", it's not guarantied that they will ever finish (thus holding the allocated memory). I suppose you could test it easily by temporary disabling all Lua scripts and see what happens in a few hours. 8 to 10% is way too much, my experience is that it will almost stay the same after a few min after starting up.

@manjh I now see that you're not the TS. You mentioned rebooting the Pi, but then we don't know if it's Domoticz causing the climb. Does restarting just Domoticz result in the same dip? In that case, try what I said above (disabling Lua).

I confirm the same issue here. See graph below. I noticed behavior first around a year ago when I was running beta's because the system stopped working and discovered the high memory consumption up to almost 100%. Then stepped over to the latest stable version back then. Still the memory was increasing.
I found that the problem was coming from the "blocky" scripting. I migrated the few scripts I had, to "LUA" and the problem was solved for month's. Suddenly, after the latest stable release(without changing anything) the memory increase started again. As you can see below. You can also see the point where I stop the event system and the mem increase stops immediately. Unfortunately the log doesn't reveal anything. I also switched on "debug" level logging but no errors are posted.
To be clear all this memory is used by the Domoticz process.

I I'm a little suspicious on the JSON virtual device updates, but not sure why this would be linked to the event system, although some events use this virtual sensors data. I use a lot of these virtual sensors which get updated by wired Arduino gateways using the JSON urls.

Buts since it seems other people experience the same issue this might not be the problem, unless they use the same setup. Anyway I will try to switch on the event system and disable the "Arduino gateway" devices to see if the mem leak stops.

Hope we can find the culprit as it looks like a long term problem.

Small update: I also switched off all the scripts, but left the event system active and still the memory increases...

I've tested some things but the only way to stop the memory increase is to disable the event system from the settings. Of course this is not a solution.
So for now I'm stuck. I narrowed it down to the event system but further analysis probably requires source code knowledge. Even disabling the scripts individually doesn't help. I'm wondering who to contact about the issue.
At the moment working on a Domoticz implementation in my new build project. Altough Domoticz is a very stable and flexible system I'm a little concerned about this persistent issue.

To be sure I deleted all scripts from the event system. Also removed all timers and notifications just to be sure. So the event system is running idle. As is visible in the attachment the memory increase continues unchanged. This time a screenshot from the Domoticz process only.